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Mineralogical Magazine; February 2006; v. 70; no. 1; p. 15-26; DOI: 10.1180/0026461067010310
© 2006 Mineralogical Society of Great Britain and Ireland
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Petrology of forearc peridotite from the Hahajima Seamount, the Izu-Bonin arc, with special reference to chemical characteristics of chromian spinel

H. Okamura1,*, S. Arai1 and Y.-U. Kim2

1 Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
2 School of Marine Science and Technology, Tokai University, 3-20-1 Shimizuorido, Shizuoka 424-8610, Japan

* E-mail: eisin{at}earth.s.kanazawa-u.ac.jp

Forearc peridotite is generally characterized by low Mg# (= Mg/[Mg + Fe2+] atomic ratio) at a given Cr# (= Cr/[Cr + Al] atomic ratio) of chromian spinel compared to common abyssal peridotite. This may be due to (1) smaller modal abundance of spinel and/or (2) lower equilibrium temperature, for the forearc peridotite. Forearc peridotite has the same amount of spinel as abyssal peridotite, eliminating the first possibility. Spinel in harzburgite and dunite from the Hahajima Seamount at the Izu-Bonin forearc, has a large Cr#, >0.57, and the Mg# is slightly variable towards low values at a given Cr#. The Mg# of spinel cores decreases strongly with a decrease in size. This is due to cooling along with hydration, which gave rise to a compositional variation of Ca-amphibole, from edenitic hornblende (high-temperature) to tremolite (low-temperature) in the Hahajima peridotite. The average two-pyroxene temperature of the Hahajima peridotite, 921°C, is less than that of the abyssal peridotite (1138°C), which is not consistent with a size-dependent Mg# of spinel for the latter. Forearc peridotite has been cooled effectively by H2O released from the subducted slab, causing a small Mg# of their spinels.

KEYWORDS: forearc peridotite, chromian spinel, Mg#, cooling, grain size




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 J PetrologyHome page
S. Arai and S. Ishimaru
Insights into Petrological Characteristics of the Lithosphere of Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review
J. Petrology, April 1, 2008; 49(4): 665 - 695.
[Abstract] [Full Text] [PDF]


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